1. Field of the Invention
This invention relates generally to an air delivery sub-system for a fuel cell system and, more particularly, to an air delivery sub-system for a fuel cell system, where the air delivery sub-system uses a compressor map to control the compressor speed to prevent the compressor from approaching a surge condition.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is disassociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The combination of the anode, cathode and membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation. These conditions include proper water management and humidification, and control of catalyst poisoning constituents, such as carbon monoxide (CO).
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. The fuel cell stack receives a cathode charge gas that includes oxygen, and is typically a flow of forced air from a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives a hydrogen anode input gas that is combined with the charge air to generate the power, as discussed above.
It is known in the art to employ a turbo-machine type compressor, such as a centrifugal, radial, axial, mixed flow, etc., compressor, in a fuel cell system. These types of compressors are low cost and low weight, and operate with low noise as compared with the positive displacement compressors, such as twin-screw compressors, that are also sometimes employed in fuel cell systems.
It is necessary that the compressor operate on its compressor map of pressure ratio (outlet pressure/inlet pressure) versus airflow. FIG. 1 is a graph with mass flow on the horizontal axis and discharge pressure on the vertical axis showing a typical example of a compressor map 50 for a turbo-machine type compressor. The compressor map 50 includes a series of speed lines 52 that show the relationship between airflow through the compressor and the discharge pressure of the compressor at various compressor speeds. Every compressor can be mapped in this manner. The compressor map 50 is bound by a surge line 54 at which the compressor suffers from an audible flow reversion caused by excessive back-pressure. This back-pressure is generally caused by the pressure drop across the stack and a back pressure valve at the cathode exhaust of the fuel cell stack that is used to control stack humidity. In other words, excessive back-pressure from the fuel cell system could cause a compressor surge condition. This surging point or reverse flow of air through the compressor is determined by the speed or RPM of the compressor, the system back-pressure, the altitude and the temperature. The map of the pressure ratio is also bound by a choke line 56 where the maximum airflow is reached with minimal pressure for a given compressor speed.
The compressor cannot operate at relatively high pressure ratios that put the compressor into a surge condition because of severe oscillation of the airflow through the compressor that could damage the compressor. Therefore, a fuel cell system that employs a turbo-machine type compressor requires surge detection and protection that detects a reverse airflow through the compressor to prevent compressor surge. Positive displacement compressors do not surge with excessive back-pressure. Therefore, a reverse airflow through the known positive displacement compressors does not present a problem or cause compressor damage, and thus, surge detection is typically not required on fuel cell systems that employ these types of compressors.